Impulse generators of the above described type are arranged to generate torque impulses as the sealing element or elements at a certain angular position seal off a high pressure compartment from a low pressure compartment in the hydraulic fluid chamber. At each generated torque impulse the kinetic energy of the drive cylinder is transferred to the impulse receiving portion of the output shaft via the sealing element or elements and a high pressure peak built up in the high pressure compartment. This results in an almost instantaneous stop of the rotation of the drive cylinder relative to the output shaft whereby the kinetic energy of the drive cylinder is transferred to the output shaft. Although the driving torque of the motor continuously acts on the cylinder, acceleration of the drive cylinder to generate another impulse is initially hindered in that the hydraulic pressure in the high pressure compartment only slowly decreases via leakage clearances past the sealing elements. This means that the impulse rate of the impulse generator becomes rather low. If the leakage clearances were widened between the sealing element or elements and the cylinder it would be possible to shorten the stop phases of the cylinder after each impulse has been delivered and thereby increase the impulse rate. However, widened leakage clearance would also lead to that the high pressure pulses would be severely reduced in magnitude and, although the impulse rate is increased, the efficiency of the impulse generator would be limited in an undesirable way.
In a previously known way, described in U.S. Pat. No. 4,735,595, the impulse rate of a torque impulse generator is increased in that a spring type valve element is arranged to open up a by-pass connection between the high pressure compartment and the low pressure compartment immediately after a high pressure pulse has been generated. This means that there will be no remaining pressure in the high pressure compartment to hinder or delay a quick acceleration of the drive cylinder before a next coming impulse, and hence limit the impulse rate of the impulse generator.
In a similar way, described in U.S. Pat. No. 3,233,537, a spring biased by-pass valve device in the drive cylinder is arranged to drain the remaining pressure in the high pressure compartment as soon as an impulse has been delivered to thereby increase the impulse rate of the torque impulse generator.
Both the above related prior art devices suffer from a drawback in that they are pressure activated, which means that they start closing the by-pass connection as the pressure in the high pressure compartment reaches a certain level. This means that they are still open during the initial part of the pressure build-up in the high pressure compartment, which means that a certain part of the hydraulic fluid in the high pressure compartment will be communicated to the low pressure compartment before the high pressure compartment is fully closed and the impulse generating high pressure pulse is to be created. The result is an undesired reduction of the accomplished pulse pressure in the high pressure compartment and hence an impaired torque impulse force. In the same way this known type of valve device does not open up a by-pass communication until the pressure in the high pressure compartment has decreased to a certain level which means an undesired delay in the acceleration before the next coming torque impulse.